JPH0647180B2 - Manufacturing method of stainless clad steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a continuous production method for a thin stainless clad steel having a good yield and good productivity.

[Prior Art] Conventionally, most of thin stainless clad steels have been manufactured by an assembly rolling method, an explosion rolling method, or a casting rolling method. However, these methods have problems in that the pretreatment of the raw material is complicated and the yield is low, so that it is impossible to manufacture them sufficiently cheaply as compared with the solid stainless steel material.

The outline and problems of each of these conventional methods will be described in more detail below.

(a) Assembly rolling method Laminate the stainless steel plate and the base steel plate of the laminated material,
This is a method in which the air between the two is removed, then the entire circumference is welded, and this is heated and rolled into a product. According to this method, a good product with stable clad can be obtained, but it takes a lot of time to carry out welding, it is necessary to cut off the periphery after rolling, and further, the first half is for rolling in a sheet state, There are problems such as poor productivity.

(b) Bomb-rolling method First, a normal clad-cladding method is used to obtain a thick clad material of stainless steel and a base steel material.
Similar to (a), this is a method of heating and rolling the product into a product. In this case as well, a product of relatively good quality can be obtained, but problems can be pointed out such as the fact that the cost of bombardment itself is considerably high and the productivity is poor because the rolling is done in the sheet state in the first half.

(c) Casting rolling method For example, in the case of a three-layer clad material, the base material is set in the center of the mold, and the molten stainless steel is poured downward and cast to first produce a clad steel ingot. Alternatively, stainless steel may be set in a mold and molten base metal may be cast. Then, this is subjected to slab rolling, hot rolling and, if necessary, cold rolling to obtain a product. In this case, there are problems that it is difficult to keep the clad ratio constant due to melting during casting, and the yield is poor because the head, bottom and side surfaces of the steel ingot are cut off in large amounts.

[Problems to be solved by the invention] In order to improve the productivity and the yield, the rolling clad method in which the coils are used together is optimal, and in order to increase the bonding strength between the base material and the laminated material, hot rolling is required. The rolling method is more preferable.
By the way, in order to handle with a coil, there is a limit to the plate thickness of both the base material and the laminated material, and as the thickness increases, it becomes difficult to supply the roll with the coil. Further, when attempting to manufacture inexpensive stainless clad steel, it is necessary to keep the clad ratio as small as possible in order to save the material cost.

In consideration of the above conditions, when an inexpensive stainless clad steel is manufactured by the hot rolling method of the same coil, the plate thickness of the laminated material is inevitably thin. For example, the coil has a clad ratio of 10% on a 4.5 mm thick base material that is easy to handle.
So, when laminating stainless steel, which is a laminated material, the thickness of the laminated material is 0.5 mm, which is considerably thin. Even if an attempt is made to process such a combination into a clad material by a hot rolling method, there is a problem in that pressure welding is impossible, or sufficient bonding strength cannot be obtained even if pressure welding is performed.

The present inventors have conducted research to solve such problems of the hot rolling clad method for thin plate materials, and as a result, used a relatively thick laminated material in which the temperature decrease at the joint interface during rolling can be ignored. As a result of the hot rolling clad test, in order to obtain sufficient bonding strength by pressure welding, it is necessary to give a certain rolling reduction, and the rolling reduction becomes smaller at higher temperatures.

The reason why pressure bonding is impossible or sufficient bonding strength is not obtained even if pressure bonding is performed is that when the laminated material is thin, the contact interface heat conduction from the laminated material to the roll causes the temperature to drop significantly at the bonding interface. In order to prevent the temperature drop at the joining interface, the rolling speed should be adjusted according to the plate thickness of the laminated material so that the contact time between the material to be rolled and the roll until the reduction ratio required for the above-mentioned pressure welding is given becomes short. The inventors of the present invention have found that sufficient bonding strength can be obtained by strictly controlling the rolling conditions such as.

[Means for Solving Problems] <Hot Rolling Clad Condition under Conditions in which Temperature Decrease at Bonding Interface can be Ignored> The present inventor uses austenitic stainless steels SUS304 and ferritic stainless steels as a bonding material. SUS4
44. Mild steel was selected as the base material, and hot clad rolling experiments were conducted under various conditions using the test piece (a) and the experimental apparatus (b) shown in FIG. The rolling diameter of the rolling mill is 210 mm.
Therefore, Ar gas was used as the shield gas during heating.

Thickness of laminated material 2mm, thickness of base material 4mm, rolling speed 0.2m / s
A hot rolling clad test was carried out at ec, and the adequacy and degree of joining were arranged according to the relationship between the heating temperature and the rolling reduction and shown in FIG.
When the laminated material is SUS304, the heating temperature is T (° C), and the rolling reduction is r. Higher temperature than the straight line AB indicated by r = -8 × 10 ^-4 × T + 0.99 (3) It has been found that it is possible to press-bond with and the sufficient bonding strength can be obtained.

As for SUS444, as shown in Fig. 2, it should be possible to perform pressure welding at a higher temperature and a higher pressure reduction rate than the straight line CD located slightly below the straight line AB in the case of SUS304, and to obtain sufficient bonding strength. I understood. Therefore, in any case, it is possible to join at a temperature higher than the straight line AB shown by the equation (3) and at a high pressure lower ratio side, and sufficient joining strength can be obtained.

Furthermore, when the rolling speed is reduced from 0.2 m / sec to 0.1 m / sec, or when only the thickness of the laminated material is reduced from 2 mm to 1 mm without changing the rolling speed, that is, the material to be rolled during rolling. The same experiment was performed under the condition that the joint interface is more likely to deteriorate due to heat conduction from the roll to the roll.
It was confirmed that the relationship between the heating temperature for welding and the rolling reduction for SUS444 and SUS444 was almost the same as the straight line AB and the straight line CD in FIG. 2 for each steel type. Therefore, the rolling conditions for obtaining the above-mentioned FIG. 2, that is,
When the thickness of the laminated material is 2 mm and the rolling speed is 0.2 m / sec, the temperature drop due to heat conduction does not reach the joint interface within the contact time between the material to be rolled and the roll, and Fig. 2 shows such a temperature drop. It can be seen that the joining conditions are shown under the condition that no occurrence of.

<Manufacturing conditions for rolled clad to prevent temperature decrease at the joint interface> Temperature changes at the joint interface during hot rolling are (a) temperature decrease due to contact heat conduction between work and roll, and (b) plastic working Strictly speaking, it is quite complicated with the phenomenon of heat generation due to the rising phenomenon of the interface. However, the inventors consider that the factor (a) is the most important in rolling particularly when the plate thickness of the laminated material is thin, and further, as a first approximation, the unsteady state in the state in which the shape change during rolling is ignored. We considered the problem as a phenomenon of heat conduction, and then confirmed the validity of such an assumption by experiments.

Based on the above assumptions, further, assuming that the surface of the material to be rolled at the time of contact between the material to be rolled and the roll is maintained at a certain constant temperature between the heating temperature of the material to be rolled and the roll surface temperature,
By considering the unsteady heat conduction in the direction perpendicular to the joint interface, in order to suppress the temperature decrease at the interface, C ′ should be a constant, It is expected that the condition of will be satisfied. However, in the equation (4), h: sheath thickness m α; thermal diffusion coefficient of the composite material m ² / sec, the density of the composite material is ρ, the specific heat is C, and the thermal conductivity is k,
It is given by α = K / ρC.

t: contact time between rolled material and roll sec The contact time t is approximately given by the following equation from geometric analysis.

However, in the formula (5), R is roll radius, mr is reduction ratio, H is total thickness of material to be rolled, mV is rolling speed, and m / sec. When the reduction rate r is larger than the reduction rate indicated by the straight line AB shown in FIG. 2, the reduction rate for each heating temperature indicated by the straight line AB is adopted instead of the actual reduction rate. This is because what is now a problem is the temperature decrease until the reduction ratio necessary for bonding is given, and the temperature decrease after bonding does not pose a problem. The following equation is obtained from equations (4) and (5).

Strictly speaking, the thermal diffusion coefficient changes with temperature, but it is considered to be almost constant unless the temperature range is wide, so by replacing it with C ′ ² α = b (constant), Becomes

The present inventors have found that 0.3 mm to 1.0 mm of SUS304 and SU
S444, 4.0mm mild steel was used as the base metal, the heating temperature and rolling speed were changed, and the result of the experiment was the horizontal axis as shown in Fig. 3. When the data is arranged by taking V on the vertical axis, the joining region and the non-joining region can be divided into two by a single straight line that passes through the origin in almost any case. Therefore, as the slope of this straight line, the constant b in the equation (7) could be obtained as follows.

b = 2.2 × 10 ⁻⁶ m ² / sec Therefore, the equation (7) is as follows.

As described above, Ar gas or a reducing gas such as hydrogen gas or ammonia decomposition gas is used as the heating atmosphere, and heating is performed while preventing the oxidation of the surfaces of the joining material and the base material to be joined, and the reduction of pressure at that time is performed. It is sufficient to select the rolling rate to be larger than the rolling reduction r obtained by the equation (3) and to select the rolling speed that satisfies the equation (8) even when the laminated material is thin, for example, 0.5 mm or less. It is possible to manufacture a rolled clad material having excellent joint strength.

As described above, austenitic stainless steel SUS304 and ferritic stainless steel are used as a composite material.
SUS444 can be joined to the base material under almost the same rolling conditions. The reason for this is that, especially for SUS444, data on the physical properties at high temperatures have not been obtained in sufficient detail, but both the thermal conductivity and the specific heat, which are the basis of the thermal diffusion coefficient α that determines the thermal conductivity in Eq. (4), For both steel types, the values are almost the same near 900 ° C to 1000 ° C (see Stainless Steel Handbook, published by Nikkan Kogyo Shimbun in 1978, P. 106 to P. 108).
It is considered that α also has a value that hardly changes.

As a typical example of austenitic stainless steel and ferritic stainless steel, SUS3
Although 04 and SUS444 have been taken up and explained, other austenitic stainless steels and ferritic stainless steels have the same mechanical properties and physical properties at high temperatures as SUS304.
Alternatively, since it is essentially the same as SUS444, the present invention can include the case where these stainless steels are used as a composite material. Further, for the same reason, it can be applied to the case of so-called duplex stainless steel in which both phases of the austenite phase and the ferrite phase are mixed.

Also, as the base material, not only mild steel but also carbon steel and high-strength steel,
Alternatively, stainless steel or the like other than the laminated material can be selected.

Further, the present invention is not limited to the two-layer clad steel in which the laminated material is arranged on only one side of the base material, and can be adopted as a method for producing a three-layer clad steel in which the laminated material is arranged on both sides. In that case, if When the plate thickness of the laminated material is different, the lower limit of the rolling speed may be obtained by using the thinner plate thickness by the formula (2).

Example of the Invention The method according to the present invention was carried out using the apparatus shown in FIG. The heating was resistance heating by direct energization of the material to be rolled, and an ammonia decomposition gas was introduced into the furnace in order to prevent oxidation of the joint surface. The roll diameter is 210 mm. Using such an apparatus, an attempt was made to produce a three-layer clad steel by using 4.0 mm thick mild steel SPCH as a base material and two 0.5 mm thick SUS304 stainless steels as combined materials. The heating temperature is 1000 ° C, and the rolling reduction and rolling speed are (1)
The values are 0.30 and 0.2 m / sec, which satisfy the equations and (2).

The three-layer hot-rolled clad material produced under the above conditions had sufficient bonding strength, and could be easily cold-rolled to 1.0 mm at a total reduction of about 70% without intermediate annealing. .

About a part of the clad material thus obtained, 1000
A heat treatment was carried out at a temperature of 10 ° C. for 10 minutes, followed by quenching, and as shown below, a joint strength test, a tensile test, a bending test and a microstructure observation of the cross section were performed.

To measure the bonding strength between the laminated material and the base material, first, clad test pieces of about 10 mm square were cut out, and the two materials that were used as holders for attaching to the tensile tester were attached to both sides of the laminated material and had a diameter of 10 mm and a length of about 10 mm. 80 mm end faces were joined with silver brazing to obtain a test material. Using such a test piece, a tensile test was performed using an Instron tester to examine the strength in the direction perpendicular to the joint surface. As a result, in all the test pieces, the brazed portion was fractured by a stress of 29 kg / mm ² or more, and the strength of the joint portion between the laminated material and the base material had sufficient strength. Was known.

Next, as shown in Table 1, the results of the tensile test conducted using JIS Z 220113B test pieces are shown in Table 2 as reference values.
It was found that it has characteristics comparable to those of JIS standard values of SUS304. In addition, the results of the bending test conducted at a bending radius of 4 mm showed that no peeling or cracking was observed at the joint interface, indicating that it had sufficient bending ductility.

Further, the structure of the cross section of the clad material is as shown in FIG.
Precipitates such as carbides were not found in the vicinity of the joint interface, and the structure was sound.

Example 2. Ferritic stainless steel S with a thickness of 0.5 mm as a laminated material
US444, using mild steel SPCH having a thickness of 4 mm as a base material, under the same hot-rolling cladding conditions and cold-rolling conditions as in Example 1,
A 1.0-mm thick 3-layer clad material was obtained. However, in this case,
The final heat treatment condition before the test was 900 ° C. × 10 minutes and rapid cooling.

With respect to the clad material produced in this manner, a joint strength test, a bending tensile test, a bending test, and a microstructure observation of a cross section were performed in the same manner as in Example 1.

The bond strength test result is 27 kg for all test pieces.
It was found that the brazed part broke at a stress of / mm ² or more, and the strength of the joint between the laminated material and the base material was sufficiently high.

As shown in Table 3, the result of the tensile test was found to have properties comparable to the JIS standard values of SUS444 shown in Table 4 for reference.

Further, as a result of the bending test, no peeling or cracking of the joint interface was observed, and the bending ductility was sufficient.

In addition, as a result of microscopic observation of the cross section of the clad material, no precipitates such as carbides harmful to the workability and mechanical properties were observed in the vicinity of the joint interface.

[Effects of the Invention] As described above, according to the present invention, even if a base material steel plate having a thickness of 6 mm or less that is easy to handle in a coil state is used, a stainless steel laminated material that is sufficiently thin is selected according to the thickness. Since it is possible to produce stainless clad steel with a small clad ratio with high yield and high efficiency,
It is extremely effective in reducing costs.

Further, according to the present invention, as compared with the assembly rolling method, the explosion-bonding rolling method, and the casting rolling method, the joining material and the base material are bonded, and the time at which they are held at high temperature is extremely short. Since the diffusion of C into stainless steel hardly occurs, the precipitation of Cr-based carbide, which is harmful to the mechanical properties and workability of the clad material, is suppressed in the vicinity of the joint interface, and thus the stainless clad steel has excellent properties. Can be manufactured.

[Brief description of drawings]

FIG. 1 is a view showing the shape of a test piece used to determine a joining condition in a hot rolling clad test and an outline of a test apparatus. FIG. 2 is a diagram showing the relationship between the heating temperature and the reduction rate at which joining is possible in the case where the laminated material is SUS304 and SUS444 having a thickness of 2 mm. Figure 3 shows the rolling speed V when the laminated materials were SUS304 and SUS444. The figure which shows the area | region which can be joined and arranged by the relationship of. FIG. 4 is a schematic diagram of the hot rolling clad manufacturing apparatus used in the examples of the present invention. FIG. 5 is a view showing a metallographic structure of a cross section in the vicinity of the bonding interface of SUS304 stainless clad steel manufactured by the method of the present invention. 1, 1a, 1b ... Laminated material, 2 ... Base material, 3 ... Dummy plate, 4 ... Ar gas introduction hole, 5 ... Heating furnace, 6 ... Oxidation preventing heating jig, 7 ... Hot Rolling rolls, 8a, 8b ...
Laminated Uncoiler, 9 …… Base Material Uncoiler, 10…
… Atmosphere direct current heating furnace, 11 …… Ammonia decomposition gas introduction hole, 12 …… Clad material coiler.

Claims (4)

[Claims] 1. When manufacturing a clad steel using stainless steel as a composite material by a hot rolling method, the composite material and the base material heated in an inert gas atmosphere or a reducing gas atmosphere are heated at a heating temperature of T. (° C.) Rolling is performed at a rolling reduction r or more obtained from r = −8 × 10 ⁻⁴ × T + 0.99 (1), roll radius is R (m), and total thickness of rolled material Where H (m) is the thickness, h (m) is the thickness of the stainless steel of the laminated material, and r for each heating temperature determined by equation (1) is used. A method for producing a stainless clad steel, which comprises rolling at a rolling speed V (m / sec) or more obtained from the following relational expression. 2. The method for producing a stainless clad steel according to claim 1, wherein the laminated material is austenitic stainless steel. 3. The method for producing a stainless clad steel according to claim 1, wherein the laminated material is ferritic stainless steel. 4. The austenite / ferrite 2 composite material.
The method for producing a stainless clad steel according to claim 1, which is a duplex stainless steel.